1. Field of the Invention
The present invention relates to a head for a thermally assisted magnetic recording device and a thermally assisted magnetic recording device.
2. Description of the Related Art
Recently, there has been proposed a thermally assisted magnetic recording method as a recording method achieving a recording density of not less than 1 Tn/in2 (H. Saga, H. Nemoto, H. Sukeda, and M. Takahashi [1999], “J. Appl. Phys”. 38, Part 1, 1839, Japan). In a conventional magnetic recording device, there is a problem that recorded information is lost due to thermal fluctuations when a recording density becomes not less than 1 Tn/in2. To prevent this, it is necessary to increase coercivity of a medium. However, since the intensity of a magnetic field which can be generated is limited in size, it becomes impossible to form a recording bit on the recording medium when the coercivity is excessively increased. To solve this problem, in a thermally assisted magnetic recording method, at the right moment of recording, the medium is optically heated to decrease the coercivity. By this, recording on a medium having high coercivity becomes possible, thereby it becomes possible to achieve the recording density of 1 Tb/in2.
In this thermally assisted magnetic recording device, it is necessary that a spot diameter of irradiating light should be comparable with a recording bit in size (several 10 nm). Because, when it is larger than that, information in an adjacent track is deleted. To heat such a minute region, an optical near-field is used. The optical near-field is a localized electromagnetic field (the light whose wave number has an imaginary number component) which is present in a vicinity of a small object whose size is not larger than the wavelength of light. The optical near-field is generated by using a minute aperture with a diameter which is not larger than the light wavelength or by using a metal scatterer. For example, there has been proposed an optical near-field generator using a metal scatterer being triangular in shape as a highly effective optical near-field generator in “Technical Digest” of 6th international conference on near field optics and related techniques, the Netherlands, Aug. 27-31, 2000, p. 55. When light is made incident on a metal scatterer, plasmon resonance is exited in the metal scatterer so that an intensified optical near-field is generated at an apex of the triangle. By using this optical near-field generator, it becomes possible to highly effectively concentrate light in a region which is not larger than several 10 nm.
In a case where a medium is heated by using an optical near-field generator using the above-described metal scatterer, light which is not irradiated on the metal scatterer is made incident on the medium as background light. The spot diameter of this background light equals to the spot diameter of the incident light, but the spot diameter of the incident light cannot be made smaller than the incident light wavelength due to a diffraction limit. Therefore, the medium is heated in the circumference of a region where the optical near-field is present, the region being several 10 nm. Consequently, there is a possibility that information in an adjacent track is deleted by the heating by this background light.